Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system may allow the fuel vapors to be purged into the engine intake manifold for use as fuel. For example, vacuum generated by the intake manifold during engine spinning may be used to draw fresh air through the fuel vapor canister to purge the stored fuel vapors into the intake manifold.
However, when an engine is boosted, it may be more difficult to purge fuel vapors from the fuel vapor canister. For example, during boosted engine operating conditions, the intake manifold pressure may be too high to generate a desired amount of flow from the fuel vapor canister to the intake manifold. Various approaches for purging a fuel vapor canister in boosted engines are known. In one example approach, fuel vapor purging during boosted conditions is carried out by utilizing one or more ejectors to generate the vacuum required for drawing fresh air through the canister. During non-boosted conditions, the fuel vapor canister is purged with fresh air by utilizing the intake vacuum.
However, the inventors herein have identified several disadvantages with such an approach. For example, due to the use of ejectors, system complexity and cost are increased. Further, during purging, some of the fresh air utilized for desorbing the stored fuel vapors is injected into the intake along with the desorbed fuel vapors. Consequently, an air-fuel ratio of the engine is altered. For example, when additional external air is introduced during purging, the amount of air entering the engine cylinders may increase. Consequently, an amount of fuel injected is increased in order to maintain the exhaust air-fuel ratio near stoichiometry. As a result, fuel consumption is increased. Further, due to increased air-flow to the engine, the resulting engine torque output may be greater than requested by the vehicle operator. In order to compensate for increased torque due to the increased airflow, a vehicle controller may adjust one or more engine actuators (e.g., retard spark timing from MBT) to reduce torque. Such measures for torque compensation may degrade the engine efficiency.
In one example, some of the above issues may be at least partly addressed by a method for a boosted engine, comprising: during purging a fuel vapor canister, during a boosted condition, flowing compressed air from a first intake passage downstream of a compressor into the canister, and delivering the purge gases to a second intake passage upstream of the compressor; and during a non-boosted condition, flowing intake air from the first intake passage into the canister, and delivering purge gases from the canister to an engine intake manifold. In this way, by purging a fuel vapor canister with air from the intake during both boosted and non-boosted conditions, a desired combustion air-fuel ratio may be maintained.
As an example, when fuel vapor purging conditions are met, the engine is operating with boost, pressure difference across the compressor may be utilized to direct compressed intake air from a first intake passage downstream of the compressor and upstream of an intake throttle into the canister and then deliver purge gases including desorbed fuel vapors and intake air from the canister to a second intake passage upstream of the compressor. If the engine is operating without boost, intake manifold vacuum may be utilized to direct intake air from the first intake passage into the canister, and then deliver purge gases from the canister to an intake manifold downstream of the engine. Further, during both non-boosted and boosted conditions, the flow of intake air from the first intake passage to the canister may be regulated by a pressure regulator in order to regulate a pressure of intake air delivered to the canister for purging. Still further, during both boosted and non-boosted conditions, the purge flow (including flow of intake air and flow of purge gases) may be directed via a common path (starting from the first intake passage, passing through the canister, and then through a purge conduit including a purge valve) until a node is reached at the end of the purge conduit and downstream of the purge valve. During boosted conditions, the flow at the node may be directed to the second intake passage upstream of the compressor via a second purge conduit. During non-boosted conditions, the flow at the node may be directed to the intake manifold downstream of the throttle via a third purge conduit.
In this way, by utilizing air from the intake to purge a fuel vapor canister during both boosted and non-boosted conditions addition of external air to the intake manifold during purging may be reduced. Consequently, a desired air-fuel ratio may be maintained. As a result, engine efficiency may be improved. Further, by eliminating the use of additional ejectors and pumps for purging, system complexity is reduced. Still further, by regulating air flow through the canister, additional structural reinforcements for the canister may not be required. As a result, system cost is reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.